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Author Topic: Image size vs crop factor  (Read 3328 times)
wynpotter
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« on: April 14, 2006, 09:37:43 AM »
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I realize this topic has been chewed on before but in a little different POV I pose this question.
If I have a 135mm lens on a 6x7 format(120film) and a normal lens is about a 105 and I crop from the middle of the film a 22mmx16mm area will the image be the same as if I had used a APC 22x16 mm digital with a lens that was about 30% more telephoto that the normal on the digital or about a 40mm.
To put it another way, if I adapt a 135mm (6x7) lens to a 22x16mm sensor size digital camera, what difference would I see in terms of actual field of view in the image.Since the image is cropped from the film at the same size as the sensor as the telephoto ratio is the same, is the image the same?
Did I miss something along the way? Wyndham
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BJL
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« Reply #1 on: April 14, 2006, 04:37:03 PM »
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If I have a 135mm lens on a 6x7 format(120film) ... and I crop from the middle of the film a 22mmx16mm area will the image be the same as if I had used a APC 22x16 mm digital with a lens that was about 30% more telephoto that the normal on the digital or about a 40mm.
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If you use the same 135mm focal length in each case, they will form equal sized images in the focal plane, and so the 16x22mm chunks of those images will cover the same field of view.

However, if you use a 40mm lens with the 16x22mm format camera, its image will cover about the same field of view as the image formed on the entire 6x7 frame by the 135mm lens, so the 16x22mm crop from 6x7 format to will cover a far narrower field of view. Effectively, you are comparing lenses of very different focal lengths (135mm vs 40mm) in the same format (16x22mm).

The only format that counts in this comparison is the height and width of the part of the image that you use (16x22mm), not any part of the image formed by the lens that is either not recorded by the film or sensor, or is recorded but then gets cropped away.
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wynpotter
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« Reply #2 on: April 15, 2006, 11:20:25 AM »
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BJL thanks and to continue a bit.
Example: if with  the 135mm on the 6x7 we shot an image and in the middle of that image is a" for sale" sign  and it measures 22x16mm in the film and you shoot the same image from the same spot with a dslr and zoomed in to cover just that "for sale" sign so it to would cover the 22x16 area of your sensor, what would the telephoto mm be on the dslr? (I sometimes flip relationships and ratios)  would I need about 400mm on the dslr.
If I know  that the diagonal of the apc sensor in about 1 inch and the diagonal of the 120 film is 3.5in  and then if I multiply 3.5 x the 135mm that will or will not give me the mm of the apc lens needed(aprox 400mm)
This might be a bit convoluted, so thanks for the any help you might give. Wyndham
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DarkPenguin
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« Reply #3 on: April 15, 2006, 12:11:06 PM »
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Unless I'm misreading your question I think they would be the same.  I mean since you reduced your film down to the same crop as the DSLR there shouldn't be any difference.
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AJSJones
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« Reply #4 on: April 15, 2006, 02:23:51 PM »
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BJL thanks and to continue a bit.
Example: if with  the 135mm on the 6x7 we shot an image and in the middle of that image is a" for sale" sign  and it measures 22x16mm in the film and you shoot the same image from the same spot with a dslr and zoomed in to cover just that "for sale" sign so it to would cover the 22x16 area of your sensor, what would the telephoto mm be on the dslr? (I sometimes flip relationships and ratios)  would I need about 400mm on the dslr.
If I know  that the diagonal of the apc sensor in about 1 inch and the diagonal of the 120 film is 3.5in  and then if I multiply 3.5 x the 135mm that will or will not give me the mm of the apc lens needed(aprox 400mm)
This might be a bit convoluted, so thanks for the any help you might give. Wyndham
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As BJL said, the only thing you need to think about is what ACTUALLY gets recorded

"The only format that counts in this comparison is the height and width of the part of the image that you use (16x22mm), not any part of the image formed by the lens that is either not recorded by the film or sensor, or is recorded but then gets cropped away."

AKA Don't think that the 16x22mm part of a 6x7 film frame behaves any differently than the 16x22mm part of an 8x10 view camera sheet or the 16x22mm part of an apc sensor.  

For any given lens (such as your 135mm) and camera position, if the for sale sign in the distance fills the 16x22mm part of the 6x7 film frame, it MUST also fill the 16x22mm  part of the 8x10 or the 16x22mm  apc ( for simplicity we're not quibbling about aspect ratio differences here).  The lens will be the same distance from each of the three capture surfaces no matter how big they are .  So, when you look at the ACTUAL focal length on the DSLR lens after framing the for sale sign, it has to be the same as on the 6x7 i.e. 135 mm - you do NOT need the 400.

In your example where you think about a fixed 16x22 mm image area , you are confusing yourself by thinking about diagonal ratios.    You only need those concepts when you think of the "For sale" sign FILLING the 6x7 or 8x10 sheet.  Example : If your camera position is fixed and you want to capture apc, 6x7 and 8x10 images of only the sign, THAT's when you need different focal length lenses for the different formats.
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wynpotter
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« Reply #5 on: April 15, 2006, 02:51:29 PM »
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Thanks, I do tend to put apples and grapefruit in the same basket at times and befuddle myself. Wyndham
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